program t;
var n:int64;
f1,f2:text;
begin
  assign(f1,'input.txt');
  assign(f2,'output.txt');
  reset(f1);
  rewrite(f2);
  read(f1,n);
  write(f2,n*(1+abs(n))div 2+ord(n<1));
  close(f1);
  close(f2);
end.
